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In-Stream Wetland Improves Water
Quality

An in-stream wetland removes excess nitrogen from a stream in the Herrings
Marsh Run watershed in Duplin county, North Carolina.

More and more newspapers are reporting concern about a major water pollution
problem in the eastern United States--nonpoint source nitrogen in rivers.
Numerous rivers and major bodies of water, including the Chesapeake Bay and the
Gulf of Mexico, are plagued by high amounts of this nutrient.

Some of the nitrogen polluting these waterways comes from crop fields and
grazinglands. Both natural and agricultural ecosystems have a certain amount of
nutrient loss. That's because rainfall dissolves and leaches nitrogen and other
nutrients from soil into nearby streams and rivers that eventually feed coastal
bodies of water.

Nitrogen stimulates growth of aquatic flora and microbes, which often lower
the amount of dissolved oxygen in the water. With little oxygen to sustain
them, native fish and shellfish that are economically and environmentally
important die or are displaced.

"We know that natural wetlands are one of the most effective ecosystems
for transforming and removing agricultural and industrial nitrogen and other
chemicals from water," says Agricultural Research Service soil
scientist Patrick G. Hunt. "Wetlands are an excellent last line of defense
for water quality."

Hunt evaluated the ability of a constructed in-stream wetland to remove
nitrogen from a nitrogen-contaminated stream in the Herrings Marsh Run
watershed in Duplin County, North Carolina. He worked with agricultural
engineer Frank J. Humenik at North Carolina State University in Raleigh, along
with agricultural engineer Kenneth C. Stone, soil scientist Terry A. Matheny,
and agricultural engineer Melvin H. Johnson at the ARS Coastal Plains Soil,
Water, and Plant Research Center in Florence, South Carolina. It was part of a
USDA Water Quality Demonstration Project in the Coastal Plain of North
Carolina.

"A wetland can be constructed when the landscape offers the opportunity
for establishing a water-control structure," says Hunt. "Herrings
Marsh Run provided us with good potential for establishing a wetland and
cleaning up one of its streams.

"In this case, establishment was easy. After we stabilized the walls of
a beaver dam, we created a 60- by 600-yard wetland. The water depth at the
lower end was about 6 feet, but much of the area was less than 2.

"About 40 percent of the wetland's surface area was covered by aquatic
weeds," Hunt says. "Another 40 percent was dominated by a perimeter
of trees--swamp tupelo, red maple, and black willow. The remaining 20 percent
was mostly open water."

The wetland was only 8 acres--less than 1 percent of the watershed that
drained through it. Nevertheless, it dramatically lowered stream nitrate-N by
2.8 pounds per acre a day.

"That's about 50 percent of the nitrogen entering the wetland in a
year," says Hunt.

The nitrate was likely being changed by bacteria to gaseous nitrogen through
a process known as denitrification. Such denitrification was favored by the
low-oxygen conditions in the wetland. Dissolved oxygen was generally less than
50 percent saturation throughout the wetland waters, and the sediment was
dominated by an absence of oxygen.

Water entered the wetland at an elevated nitrate-N level--typically about 7
parts per million (ppm). During warmer months, it left the wetland with less
than 1 ppm. In cooler months, when bacterial activity was lower, nitrate levels
left the wetland as high as 5 ppm, but monthly nitrate levels were always lower
in discharge water than in inlet water.

"Wetlands are a good complement to other best-management practices for
improving water quality," says Hunt. "They are relatively inexpensive
to build, simple to operate, aesthetically pleasing, and attractive to a
variety of wildlife."--By Hank
Becker, Agricultural Research Service Information Staff.